String of pearls

Identification and origin

Curio rowleyanus belongs to the daisy family, Asteraceae. Long known as Senecio rowleyanus, it was recently moved into the genus Curio, but it is still often sold under its old name. Its epithet honors Gordon Rowley, a British botanist who specialized in succulents and cacti. It is native to dry areas of the Cape Provinces of South Africa, where it often grows in the shelter of shrubs and rocks, protected from the strongest sun.

It is an evergreen succulent perennial with weak, shallow roots. Its thin stems creep along the ground or trail 3 ft or more; where they touch soil, they root at the nodes and form dense mats. In a pot, the clump grows 12 in – 2 ft tall. The alternate leaves are lime-green spheres 0–0 in across, ending in a small point. Their round shape reduces the surface exposed to dry air and so limits water loss; their core is made of water-storage cells. A thin, darker, translucent band runs along one side: this “window” lets light into the leaf and increases the area available for photosynthesis.

In summer, small white brush-like flowers about 0 in across, with red stamens and bright yellow anthers, open at the tips of stalks 2 in long. They are inconspicuous but smell of cinnamon. Indoors the plant rarely blooms; a cool, fairly dry winter can encourage flowering. The seeds carry a cottony tuft that the wind spreads.

Light

Give it bright, filtered light, for example near an east- or west-facing window. It tolerates some direct sun, but keep it out of the midday summer sun, which scorches the leaves; a south-facing spot is not ideal. Variegated forms are even more prone to scorch.

With too little light, the stems stretch and thin out, and the pearls stay small and widely spaced.

Watering

Let the mix dry out between waterings, then water moderately. The best signal comes from the leaves: when the pearls start to look slightly shriveled, the plant is thirsty. During the growing season, from spring to late summer, water more regularly; in winter, water very sparingly, just enough to keep the leaves from shriveling.

Avoid splashing the pearls and never leave water standing in the saucer. Rainwater or filtered water is ideal, although tap water is tolerated. There is no need to mist it: it likes good air flow, but not cold air currents. Overwatering is the most common cause of death.

Potting mix

Use a cactus and succulent mix, or lighten ordinary potting mix with mineral materials (fine gravel, coarse sand, pumice), up to half and half. A blend of two parts loam-based, peat-free potting mix to one part horticultural grit also works.

Choose a shallow pot with drainage holes: the short roots cannot use a large volume of soil, which would stay wet too long. Terracotta, which lets moisture escape through its walls, is better than plastic or glazed ceramic. It is not a heavy feeder: during the growing season, a general-purpose liquid fertilizer at half strength, no more than once a month, is enough.

Temperature

It grows best at around 70–81 °F in summer. In winter, a cool rest at around 50–61 °F, with less water, suits it. It dislikes cold air currents and temperatures below 50 °F; keep it away from radiators as well.

It does not tolerate frost and can be grown outdoors year-round only in USDA zones 9b to 11b. Elsewhere, it can spend the summer outside in part shade, gradually acclimated to the sun and sheltered from heavy rain; bring it back in before the first frost.

Repotting

A newly bought plant can usually stay in its pot for at least a year. After that, repot in spring every one or two years into a pot only slightly larger: too much mix stays damp long after watering and encourages root rot. Repot right away if the mix looks very dark and waterlogged.

Pruning

Cut weak or thin, straggly stems back to the base to encourage new growth. Stems that are too long can be shortened with scissors: they soon regrow, and the trimmings make cuttings. Wear gloves, because the sap irritates the skin.

After a few years, the plant tends to decline: rather than trying to revive it, start over from cuttings. To show off its habit, grow it in a hanging basket, on a high shelf or in a shallow dish where it can creep as it does in the wild.

Propagation

• Stem cuttings: the simplest method. Cut a stem tip 3–4 in long, strip three or four pearls from the base, then lay it on moist mix or press it in lightly, covering the last few nodes. Roots form at each node in about a month. Until it roots, mist the surface instead of watering.
• Layering: lay a stem across a nearby pot of mix and pin it with a bent wire staple; detach it once rooted. Looping a long stem back into its own pot fills out a sparse clump.
• Single pearls: laid on the mix, they sometimes root, but more slowly and less reliably.
• Seed: possible, but seed is rarely available.

Relatives and cultivars: ‘Variegatus’, with green-and-cream pearls; Curio herreianus, with slightly larger, oval, striped leaves; Curio radicans, with banana-shaped leaves; Curio peregrinus, with branched, dolphin-shaped leaves.

Common problems

• Rotting stems and roots: overwatering or mix that stays too wet.
• Shriveled pearls: lack of water, but also overwatering or unsuitable light.
• Scorched or bleached pearls: direct sun that is too strong, or a move outdoors without acclimation.
• Long, bare stems and small pearls: too little light.
• Mealybugs, aphids, whiteflies, spider mites: check the stems and the center of the clump; yellow or grayish leaves can be a sign of insects.

A symptom can have several causes: before watering a plant with shriveled pearls, check the condition of the mix and the stems first.

Toxicity

The plant contains pyrrolizidine alkaloids. It is toxic to people, dogs, cats and horses; if eaten, it mainly causes vomiting, diarrhea and lethargy. The sap can irritate the skin and cause a rash in sensitive people: wear gloves when handling it. Hanging high, it is easier to keep out of reach of children and pets; if any is eaten, call a veterinarian or a poison control center.